Process for the production of advanced NF4+ salts

ABSTRACT

An improved process for the production of advanced NF 4   +  salts is described utilizing graphite salts as an HF and oxidizer resistant anion exchange medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for the production of advanced NF₄ ⁺ salts from NF₄ SbF₆. Advanced NF₄ ⁺ salts are particularly useful for applications, such as solid propellant NF₃ -F₂ gas generators, high detonation pressure explosives, and fluorinating agents for aromatic compounds.

2. Description of the Prior Art

The NF₄ ⁺ cation is unique since it combines high oxidizing power with unusual kinetic stability. Furthermore, its synthesis requires unconventional methods. Generally, complex fluoro cations are prepared by F⁻ abstraction from a parent molecule. In the case of NF₄ ⁺, however, the corresponding parent molecule, NF₅, does not exist. The chemical synthesis of NF₄ ⁺ from NF₃ and F⁺ is precluded by the fact that fluorine is the most electronegative element known and, therefore, F⁺ cannot be generated by chemical means. This problem was overcome by the discovery that NF₄ ⁺ salts can be prepared from NF₃, F₂ and a strong Lewis acid in the presence of an activation energy source. Although this principle has been demonstrated for several different Lewis acids and activation energy sources, such as heating, glow-discharge, bremsstrahlung and uv-photolysis, the only efficient and economically feasible method for preparing an NF₄ ⁺ salt is based on the high temperature and high pressure reaction of NF₃ and F₂ with either SbF₅ or SbF₃. Consequently, the only readily accessible NF₄ ⁺ salt is NF₄ ⁺ SbF₆ ⁻. Since the SbF₆ ⁻ anion is heavy and of low energy, it is undesirable for most practical applications and must be substituted by lighter or more energetic anions. Whereas the replacement of a weaker Lewis acid, such as BF₃, by a stronger Lewis acid, such as PF₅, AsF₅ or SbF₅, can be readily achieved by a simple displacement reaction,

    NF.sub.4 BF.sub.4 +PF.sub.5 →NF.sub.4 PF.sub.6 +BF.sub.3

the reverse reaction, i.e. the displacement of SbF₆ ⁻ by BF₃ is not possible. Based on the state of the art, substitution of SbF₆ ⁻ in NF₄ ⁺ SbF₆ ⁻ is best achieved by metathetical processes in anhydrous HF, taking advantage of the relatively low solubility of alkali metal SbF₆ ⁻ salts. A typical example is the synthesis of NF₄ BF₄ from NF₄ SbF₆ according to: ##STR1## Among the major drawbacks of this process are: (i) the purity of the resulting NF₄ BF₄ product is only about 80-90%; (ii) the yield of NF₄ BF₄ is usually only 60 to 80% due to hang-up of mother liquor in the filter cake; (iii) cesium salts are expensive; (iv) the process must be carried out in a batch-type fashion; (v) the process requires low-temperature operations and cumbersome filtration steps.

SUMMARY OF THE INVENTION

Accordingly, there is provided by the present invention an improved process for the production of advanced VF₄ ⁺ salts wherein graphite salts can be used for exchanging the SbF₆ ⁻ anion in NF₄ SbF₆ for other, more desirable anions, such as BF₄ ⁻ or AsF₆ ⁻ according to: ##STR2##

OBJECTS OF THE INVENTION

Therefore, it is an object of the present invention to provide an improved los-cost process for the production of advanced NF₄ ⁺ salts in high purity and yield from NF₄ SbF₆ using graphite salts as an anion exchange medium.

Another object of the present invention is to provide pure NF₄ ⁺ salts, so as to avoid the need for costly recrystalization steps from exotic solvents.

Yet another object of the present invention is to provide quantitative yields of NF₄ ⁺ salts.

Still a further object of the present invention is to provide low-cost NF₄ ⁺ salts which result from the fact the expensive cesium salts are replaced by the cheaper graphite salts.

Another object of the present invention is to provide a continuous operation process.

A further object of the present invention is to provide a means for eliminating cooling or cumbersome low temperature filtration steps.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention it has been found that graphite salts can be used for exchanging the SbF₆ ⁻ anion in NF₄ SbF₆ for other, more desirable anions according to: ##STR3##

Those X⁻ anions include the advanced NF₄ ⁺ salt anions which include but are not limited to BF₄ ⁻, SiF₆ ⁻², PF₆ ⁻, GeF₅ ⁻, AsF₆ ⁻, SO₃ F⁻, ClO₄ ⁻, GeF₆ ⁻², SeF₅ O⁻, TeF₅ O⁻, SnF₅ ⁻, BiF₆ ⁻, SnF₆ ⁻², MnF₆ ⁻², NiF₆ ⁻², TiF₆ ⁻², Ti₂ F₁₀ ⁻², IF₄ O₂ ⁻, WOF₅ ⁻, WF₆ ⁻, WF₇ ⁻, UF₅ O⁻, and UF₇ ⁻. The preferred salts include BF₄ ⁻, SiF₆ ⁻², PF₆ ⁻, GeF₅ ⁻, AsF₆ ⁻, SO₃ F⁻, ClO₄ ⁻, GeF₆ ⁻², SnF₅ ⁻, BiF₆ ⁻, SnF₆ ⁻², MnF₆ ⁻², NiF₆ ⁻², TiF.sub. 6⁻², Ti₂ F₁₀ ⁻², IF₄ O₂, UOF₅ ⁻, WF₆ ⁻, WF₇ ⁻, UF₅ O⁻, and UF₇ ⁻. The most preferred salts include BF₄ ⁻ or AsF₆ ⁻.

The preferred graphite cations C_(n) ⁺ include those wherein n is equal or greater than about six (n≳6), and the preferred range n lies between about 6 and about 12.

Since the graphite salts have structures consisting of polymeric graphite sheets with intercalated mobile monomeric anions, these graphite salts are totally insoluble in HF, but can readily release their anions in the form of soluble NF₄ ⁺ salts. Compared to the state of the art metathetical reactions, the novel process based on graphite salts offers the following significant improvements: (i) pure NF₄ ⁺ salts are obtained thus avoiding the need for costly recrystallization steps from exotic solvents, such as BrF₅ ; (ii) the yields are quantitative, i.e. no NF₄ ⁺ values are lost; (iii) the cost is lower because expensive cesium salts are replaced by the cheaper graphite; (iv) the process can be operated in a continuous manner, if desired; (v) cooling or cumbersome low-temperature filtration steps are not required.

The general applicability of this novel graphite based process was demonstrated by preparing advanced NF₄ ⁺ salts containing either tetrahedral or octahedral anions using NF₄ SbF₆ and the corresponding graphite salts as starting materials.

Using appropriate regeneration schemes, such as pyrolysis of the spent graphite salt or regeneration with Na⁺ X⁻, the following process cycles, written for illustrative purposes for NF₄ BF₄, are possible: ##STR4## As can be seen from these examples, the net reaction is the formation of NF₄ BF₄ from NF₃, F₂, and BF₃ with all by-products being completely recyclable. Prior to this invention, this reaction could only be achieved in very low yields under extreme conditions, such as low-temperature photolysis, radiolysis or glow-discharge, at a prohibitive cost.

By way of illustration example and not limitation the following two examples are given:

EXAMPLE I

A sample of C₁₁ ⁺ BF₄ ⁻ (3.08 g) prepared from graphite, BF₃ and F₂ by the method of Nikonorov, was placed into an exchange column consisting of 1/4" od heavy wall Teflon-PFA tubing. The column was soaked with liquid anhydrous HF. Then 1.6 g (4.91 mmol) of NF₄ SbF₆ dissolved in 8 ml of anhydrous HF was slowly passed at ambient temperature through the column. The column was washed with 5 ml of anhydrous HF and the HF solutions were collected in a Teflon receiver. The HF solvent was pumped off at ambient temperature, leaving behind 4.9 mmol of spectroscopically pure NF₄ BF₄ having properties identical to those reported in the literature.

EXAMPLE II

A sample of 5.0 mmol of NF₄ SbF₆ dissolved in 8 ml of anhydrous HF was passed, as described in Example I, through a column containing 3.5 g of C₉ ⁺ AsF₆ ⁻. The solid residue isolated from the collected effluent consisted of 5.0 mmol of spectroscopically pure NF₄ As₆ having properties identical to those reported in the literature.

Obviously, numerous variations and modifications may be made without departing from the present invention. Accordingly, it should be clearly understood that the forms of the present invention described above are illustrative only and are not intended to limit the scope of the present invention. 

What is claimed and desired to be secured by Letters Patent of the United States is:
 1. The method of exchanging the anion in an NF₄ ⁺ salt by ion exchange and in accordance with the following reaction: ##STR5## wherein C_(n) ⁺ is a graphite cation and X⁻ is an advanced NF₄ ⁺ salt anion.
 2. The method of claim 1 wherein X⁻ is selected from the group consisting of:BF₄ ⁻, SiF₆ ⁻², PF₆ ⁻, GeF₆ ⁻, AsF₆ ⁻, SO₃ F⁻, ClO₄ ⁻, GeF₆ ⁻², SeF₅ O⁻, TeF₅ O⁻, SnF₅ ⁻, BiF₆ ⁻, SnF₆ ⁻², MnF₆ ⁻², NiF₆ ⁻², TiF₆ ⁻², Ti₂ F₁₀ ⁻², IF₄ O₂ ⁻, WOF₅ ⁻, WF₆ ⁻, WF₇ ⁻, UF₅ O⁻, and UF₇ ⁻.
 3. The method of claim 2 wherein X⁻ is selected from the group consisting of:BF₄ ⁻, SiF₆ ⁻², PF₆ ⁻, GeF₅ ⁻, AsF₆ ⁻, SO₃ F⁻, ClO₄ ⁻, GeF₆ ⁻², SnF₅ ⁻, BiF₆ ⁻, SnF₆ ⁻², MnF₆ ⁻², NiF₆ ⁻², TiF₆ ⁻², Ti₂ F₁₀ ⁻², IF₄ O₂ ⁻, UOF₅ ⁻, WF₆ ⁻, WF₇ ⁻, UF₅ O⁻ and UF₇ ⁻.
 4. The method of claim 3 wherein X⁻ is selected from the group consisting of BF₄ ⁻ and AsF₆ ⁻.
 5. The method of claim 1 wherein n is equal to or greater than about
 6. 6. The method of claim 5 wherein n lies between about 6 and about
 12. 